there will be a lot of issues and it might bring your machine unbootable
So be prepared with a testing debian stick or cd

Here are some of the issues that could happen while upgrading the machine there are many dpkg-new files in the /etc/init.d/ folder so here are some tips bellow

Init scripts and networking systemGet rid of upstart first. I had problems with sysv-rc and needed to force reconfiguration. Some init scripts were installed with "dpkg-new" in the name, I had to rename them. As with X, I tried few solutions, this is distilled but stil redundant version: